Remove unused caps: UnsignedSupport and ImageLoadStoreSupport

Change-Id: I67c5a8c313438018ccafe5f760e291a31410e56c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/219397
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
11 files changed